What I know about Will and Would both are auxiliary modal verbs. Will we use for definite statement and Would we use for asking permission or in a request statement.
Keeping that in mind is the below statement is correct grammatically or not?

It would be great if you can change my date of interview at the end of
this month.

2 Answers
2

It will be great if you can change the date of my interview
(conditional 1). or

It would be great if you could change the date of my interview
(conditional 2).

No doubt, in conditional 2 sentences, we talk about unreal or imaginary situations. But it's also a fact that when we want to be more polite for perfectly possible plans, suggestions, or requests in the present or future, we usually use the conditional 2 sentences, especially in business correspondence.

You can use either of the sentences, but to be more polite you should use the sentence #2. I think there is no difference in meaning. It's very common to say "I would appreciate if you could ........".

I find this answer confusing. It begins with what is apparently one correct option for the sentence, then immediately says the sentence is wrong. Then it says that this same sentence is right. So now I have to go back and reinterpret the sentence to mean your sentence or the OP's sentence.
– user20792Dec 9 '15 at 18:16

It would be great if you could change the date ... A sentence with "would + bare infinitive", and a subclause with "if + past subjunctive". Mostly this sentence type is used for irreal statements as in "If I had money, I would buy a new car" referring to now. But here the would-form expresses politesness.
You should study the various types of conditional clauses.